Optical pumping magnetometers

ABSTRACT

The invention relates to optical pumping magnetometers, wherein an alkali vapor filling an absorption cell is optically pumped by means of a pumping source which provides light having two spectrum lines building up a doublet. The magnetometer according to the invention uses a cell whose walls are internally lined with a layer preserving the alignment of the alkali atoms. Improved detection of the R.F. lines is carried out with a birefringent plate converting said two spectrum lines radiations supplied by said cell with the same circular polarization into radiations rectilinearly polarized in mutually perpendicular directions.

United States Patent Bruin Mar. 28, 1972 [54] OPTICAL PUMPING 3,513,3835/1970 Hartline ..324/0.5

MAGNETOMETERS 7 Primary Examiner-Michael J. Lynch [72] lnventor: HenriBrun, Paris, France Attorney-Cushman, Darby and Cushman [73] Assignee:Thomson-CSF [57] ABSTRACT [22] Flled: 1970 The invention relates tooptical pumping magnetometers,

Appl. No.: 13,970

Int. Cl ..G0lr 33/08 Field of Search ..324/O.5; 331/3, 94; 330/43References Cited UNITED STATES PATENTS 5/1966 Ruddock ..324/0.5

PUMP/M6 S OUPCE' wherein an alkali vapor filling an absorption cell isoptically pumped by means of a pumping source which provides lighthaving two spectrum lines building up a doublet. The magnetometeraccording to the invention uses a cell whose walls are internally linedwith a layer preserving the alignment of the alkali atoms. improveddetection of the R.F. lines is carried out with a birefringent plateconverting said two spectrum lines radiations supplied by said cell withthe same circular polarization into radiations rectilinearly polarizedin mutually perpendicular directions.

5 Claims, 3 Drawing Figures msmsnmze me 3.652.926

SHEET 1 OF 2 PRIOR ART Ql/AETER W4 I/E FLA TE OPTICAL PUMPINGMAGNETOMETERS The present invention relates to optical pumping devicesfor measuring the intensity of a magnetic field. It relates moreparticularly to. alkali vapor magnetometers using an absorption cellwhose walls are internally lined with an appropriate coating; thiscoating enables the alkaliatoms which are aligned by the action of thepumping light, to-retain their alignment when they collide with thewalls of the cells. In this kind of magnetometers, the optical pumpingis efiected by a light source emitting two spectrum lines D, and D whichare circularly polarized in the same direction by means of a rectininearpolarizer and a birefringent quarter wave plate. The two lines D, and Dare additive in their effect, as far as the optical pumping isconcerned, but the variation in luminous intensity of the radiation D,and the radiation D emerging from the absorption cell, havecounteracting effects on the optical detection of the radiofrequencytransitions. To insure good optical detection sensitivity, it istherefore necessary to provide between the absorption cell and thephotoelectric transducer, an interference filter which transmits one ofthe lines unattenuated and absorbs the other line. Since the wavelengthsof the two spectrum lines are very close together, the filtering of oneline is a very delicate operation and optical detection is made lessefiicient as a consequence.

it is an object of this invention to avoid this drawback.

According to the invention, there is provided an optical pumpingmagnetometer comprising an absorption cell filled with an alkali vapor,a light source for emitting toward said cell a beam of light containingtwo lines D, and D, of the emission spectrum of said vapor, meanspositioned between said source and said cell for circularly polarizingin the same direction the luminous energies respectively correspondingto said lines D, and D,, a photoelectric transducer positioned forreceiving the light emerging from said cell and further polarizing meanspositioned on the path of said emerging light for selectivelytransmitting one of said lines D, and D toward said transducer, saidcell having walls internally lined with a layer preserving the alignmentof the alkali atoms.

For a better understanding of the invention and to show how the same maybe carried into effect reference will be made to the drawingaccompanying the ensuing description and in which:

F IG. 1 schematically illustrates the arrangement of the opticalelements of an optically pumped magnetometer of known kind.

FIG. 2 is an explanatory diagram.

F IG. 3 illustrates a birefringent plate associated with a rectilinearpolarizer; and

FIG. 4 schematically illustrates an optically pumped magnetometer inaccordance with the invention.

In FIG. 1 the optical elements of a magnetometer of known kind can beseen. Along an optical axis XX there is disposed an absorption cell 6containing an alkali element in the vapor state; the alkali element issubjected to the action of the magnetic field H being measured and tothat of an alternating field H, produced by inductors 9. The alkaliatoms contained in the cell 6 are optically aligned by means of apumping light source 1 which emits a light beam containing two veryclosely spaced spectrum lines D, and D The beam passes through arectilinear polarizer 3 and a birefringent plate of the quarter wavekind 4, so that on emerging from the plate 4, the two spectrum lines D,and D are circularly polarized in the adirection. Lenses and 7 arelocated at either side of the cell 6 for respectively receiving thelight beam issuing from the source 1 and transmitting it to aphotoelectric transducer 8 An electrical device (not shown) produces inthe inductors 9 an alternating current of frequency f by means of avariable frequency generator, whose frequency f is a function of thevoltage supplied by the transducer 8. The function of this device is tobring the frequency f into coincidence with the center frequency f, ofone of the radio frequency lines of the alkali vapor contained in thecell 6. An interference filter 2 is inserted between the transducer 8and the lens 7 in order to absorb one of the spectrum lines emitted bythe source 1. The optical alignment of the alkali atoms is promoted bythe provision of an appropriate coating on the internal walls of thecell 6, this coating prevents any misalignment of the atoms as aconsequence of collision with the walls.

in FIG. 2, the distribution of the quantum levels in sodium, can beseen; the magnetic field strength H, has been plotted on the abscissae,in order to show the Zeeman splitting of the hyperfine levels F l and F2 of the ground state 3 Si/2, and also of the levels of the excitedstate 3P1/2. The level F l of the ground state is splitted into threesub-levels the magnetic quantum numbers m of which are equal to 1,0 andl. The level F 2 is splitted into five sub-levels m 2 l, 0, l and 1. Inthe absence of any pumping light, it can be assumed that the sub-levelshave substantially the same atomic population and this is due toBoltzmann s equipartition law.

If the alkali vapor is illuminated by the spectrum line D, circularlypolarized in a direction a then optical absorption transitions willselectively take place between the sub-levels of the ground state andthose of the excited state; an atom at the sub-level m K, for example,of the level F 2 of ground state, is raised to a sub-level m k l of theexcited state, and then it fall back to the sub-levels k k 1 and k 2 ofthe ground state. This pumping process increases the population of thesub-levels F 2 m +2 at the expense of the lower sub-levels.

in FIG. 2, the populations of the sub-levels F 2, m 2, l, 0, 2, and 2,once the optical pumping has been completed, have been sketched; in thisstate, the alkali vapor has a positive polarization S,

The same thing happens if the spectrum line D,, of the pumping beam,with the same circular polarization of direction 0+ is used. It can besaid, therefore, that the two spectrum lines D, and D add their pumpingeffects, when they are polarized in the same direction.

However, as the pumping action due to the lines D, and D is not the samefor all the sub-levels, as far as the optical detection of the radiofrequency transitions is concerned, it is observed that the applicationof the field H, has different effects according the whether thetransparency of the vapor to the line D,, or to the line D, isconsidered. I

The following table indicates what are the probabilities of transitionof a sodium atom occupying one of the sub-levels of the ground state.

Taking into account the positive polarization S,, which the pumpinglight has produced in the alkali vapor, it will be seen that thesub-level E= 2 M 2 has a higher atomic population than do the lowersub-levels. The result is that the line D, polarized in the direction0+, is absorbed hardly at all, while the line D, is heavily absorbed; inother words, the most populous sub-level F 2 m 2 has a probability ofabsorption of the line D, which is substantially higher than itsprobability of absorption of the line D,. When the alternating field H,acts on the vapor and if the frequency of this field is close to thecenter frequency f, of a radio frequency transition in the vapor, thenthe tendency will be towards equilibrationof the populations of thesub-levels; the optical absorption of the spectrum line D reduces andthat on the line D, increases. Since these two mutually opposed effectsare applied to the photoelectric transducer, the resultant is anegligibly small detected signal. This difficulty can be obviated byarranging in front of the photoelectric transducer 8 an interferencefilter 2 which eliminates one of the two radiation fractions transmittedby the absorption cell. However, this technique has the drawback that inpractice it does not completely eliminate one of the two spectrum lines,since they are very close to each other.

The invention provides for the complete elimination of the undesiredspectrum line, while wholly transmitting the other line. To this end,the interference filter is replaced by the polarizer elements shown inFIG. 3.

The optical device shown in FIG. 3 is made up of a birefringent plate 11and a rectilinear polarizer 10. The neutral lines OA and OB of the plate11 are orientated at 45 to the direction of the polarizations producedby the polarizer 10; the entry face of the plate 11 receives the linesD, and D circularly polarized in the direction the thickness e of theplate is determined in such fashion that said radiation fractions leaveit with rectilinear polarizations which are at right angles to oneanother.

In particular, the radiation D, whose wavelength is A, leaves the plateill with a rectilinear polarization parallel to the direction ofpolarization of the polarizer 10; the radiation D leaves the plate II inthe form of a wave of wavelength A whose rectilinear polarization isperpendicular to the direction of polarization of the polarizer 10.

The thickness e of the plate 11 is determined as follows:

The radiation D,, on passing through the plate, experiences adifferential phase shift Ad), while the radiation fraction D experiencesa differential phase shift A The differential phase shifts must satisfythe following relationships:

A, being the wavelength in vacuum of the spectrum line D,;

A, being the wavelength in vacuum of the spectrum line D k, and k beingpositive whole numbers;

A being the difference between the extraordinary index n, and theordinary index n of the birefringent material of which the plate ismade. It will be assumed here that the entry and exit faces of the plateare parallel to the optical axis of the birefringent material.

These relationships lead to the following condition: K, x,= k x,

Knowing A, and A,, it is possible to select two whole numbers k, and kwhich will duly satisfy the same condition. It is then an easy matter tocalculate the thickness e of the plate 11.

By way of example, a magnetometer operating with cesium vapor, utilizesfor the optical pumping function, wavelengths of), 8,943 A. and A 8,521A.

By cutting the plate 11 from a quartz block whose indices n and n differby A 0.0085, it will be seen that the numbers k, 30 and k 31 willsatisfy the above described condition. Accordingly, the thickness of theplate 11, deduced from the formulas set out thereinbefore, has a value e3,008 u. The foregoing calculations show that by suitably selecting thewavelengths A, and A it is possible to define a plate thickness whichwill make it possible to convert the radiations D, and D into radiationswhich are rectilinearly polarized in accordance with two mutuallyperpendicular directions. In practice, it is not necessary for theradiation fractions emerging from the double-refracting plate to haveprecisely rectilinear polarizations; this mans that the determination ofthe numbers k, and k is made simpler by stating the condition k k, k,.One then has:

1 o +i,) in: 1

Using this latter formula, in the case of cesium one finds:

l o +i,) 4

By adopting the whole number k= l0, it will be seen that the thicknessof the plate ill can be made equal to e= 986 ,u.

In FIG. 4, the diagram of an optical pumping magnetometer in accordancewith the invention can be seen.

It comprises an absorption cell 16 containing alkali vapor, a lightsource 12 supplied by a generator 22 and emitting two optical spectrumlines D, and D capable of optically pumping Y the vapor contained in thecell 16, and a photoelectric detector 20 which picks up the lighttransmitted through the cell 16. Between the source12 and the cell 16,there is placed a rectilinear polarizer 13, a birefringent plate of thequarter wave type, 14, and a lens 15; between the cell 16 and thedetector 20, there is a lens 17, a birefringent plate 18 and arectilinear polarizer 19. The cell 16 is subjected to the effect of analternating field I-I, created by the inductors 21; the inductors 21pass a current of frequency f supplied by the circuit 23, the latterhaving a control input connected to the detector This circuit 23 isdesigned so that the frequency f coincides with the frequency f,,, thelatter characterizing one of the radio frequency transitions in thevapor. The plate 18 receives two radiation fractions D, and D circularlypolarized in the same direction, and converts them into radiationfractions which have mutually perpendicular rectilinear polarizations.The direction of polarization of the polarizer 19 is aligned parallel tothe bisector of the neutral lines of axes of the plate 18, so that onlyone of the two spectrum lines transmitted by the alkali vapor isactually transmitted right through to the detector 20.

Of course, the invention is not limited to the embodiment described andshown which has been given solely by way of example.

What is claimed is:

1. An optical pumping magnetometer comprising an absorption cell filledwith an alkali vapor, a light source for emitting toward said cell abeam of light containing two lines D, and D of the emission spectrum ofsaid vapor, means positioned between said source and said cell forcircularly polarizing in the same direction the luminous energiesrespectively corresponding to said lines D, and D a photoelectrictransducer positioned for receiving the light emerging from said cell,further polarizing means positioned on the path of said emerging lightfor selectively transmitting one of said lines D, and D toward saidtransducer, means for applying to said cell a RF magnetic field, andreadout means; said cell having walls internally lined with a layerpreserving the alignment of the alkali atoms.

2. An optical pumping magnetometer as claimed in claim 1, wherein saidfurther polarizing means comprise: a birefringent plate having an inputface for receiving the circularly polarized energy emerging from saidcell and corresponding to said lines D, and D an output face parallel tosaid input face, and two neutral axes lying in a plane parallel to saidfaces; a rectilinear polarizer positioned beyond said output face, saidrectilinear polarizer having a polarization direction at an angle of 45with respect to said axes; the distance of said faces being adjusted forconverting the circularly polarized luminous energies of lines D, and Dreceived by said input face into rectilinearly polarized energies ofperpendicular polarization directions.

3. An optical pumping magnetometer as claimed in claim 2, wherein saidplate is cut in a birefringent material having an ordinary and anextraordinary index of refraction differing from each other by thequantity (3,; said lines D, and D having respective wavelengths A, and Asaid distance being substantially equal to both (k,-%) and k, and kbeing two positive whole numbers.

4. An optical pumping magnetometer as claimed in claim 3, wherein saidnumbers k, and k, are equal.

5. An optical pumping magnetometer as claimed in claim 3, wherein saidalkali vapor is cesium and said birefringent material is quartz.

1. An optical pumping magnetometer comprising an absorption cell filledwith an alkali vapor, a light source for emitting toward said cell abeam of light containing two lines D1 and D2 of the emission spectrum ofsaid vapor, means positioned between said source and said cell forcircularly polarizing in the same direction the luminous energiesrespectively corresponding to said lines D1 and D2, a photoelectrictransducer positioned for receiving the light emerging from said cell,further polarizing means positioned on the path of said emerging lightfor selectively transmitting one of said lines D1 and D2 toward saidtransducer, means for applying to said cell a R F magnetic field, andreadout means; said cell having walls internally lined with a layerpreserving the alignment of the alkali atoms.
 2. An optical pumpingmagnetometer as claimed in claim 1, wherein said further polarizingmeans comprise: a birefringent plate having an input face for receivingthe circularly polarized energy emerging from said cell andcorresponding to said lines D1 and D2, an output face parallel to saidinput face, and two neutral axes lying in a plane parallel to saidfaces; a rectilinear polarizer positioned beyond said output face, saidrectilinear polarizer having a polarization direction at an angle of 45*with respect to said axes; the distance of said faces being adjusted forconverting the circularly polarized luminous energies of lines D1 and D2received by said input face into rectilinearly polarized energies ofperpendicular polarization directions.
 3. An optical pumpingmagnetometer as claimed in claim 2, wherein said plate is cut in abirefringent material having an ordinary and an extraordinary index ofrefraction differing from each other by the quantity Delta n; said linesD1 and D2 having respective wavelengths lambda 1 and lambda 2; saiddistance being substantially equal to both (k1 - 1/4 ) lambda 1 and , k1and k2 being two positive whole numbers.
 4. An optical pumpingmagnetometer as claimed in claim 3, wherein said numbers k1 and k2 areequal.
 5. An optical pumping magnetometer as claimed in claim 3, whereinsaid alkali vapor is cesium and said birefringent material is quartz.